1. Field of the Invention
The present invention relates to a magnetic recording medium, a magnetic recording and reproducing apparatus, a magnetic recording method and a magnetic reproducing method.
Priority is claimed on Japanese Patent Application No. 2013-143667, filed on Jul. 9, 2013, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, the range of application of magnetic recording and reproducing apparatus such as hard disk drives (HDDs) has remarkably increased, the apparatus has increased in importance, and, as for magnetic recording mediums used in such magnetic recording and reproducing apparatus, a remarkable improvement in recording density has been achieved. Particularly, ever since MR heads and the PRML technique were introduced, the rate of improvement in the surface recording density has increased. GMR heads, TMR heads and the like have recently been introduced, and the rate of increase has continued at a pace of 40% per year.
Therefore, as for magnetic recording mediums, there will be a demand for a higher recording density to be achieved in the future by realizing a magnetic layer with a higher coercivity, a higher signal-to-noise ratio (high SNR), and a higher resolution.
On the other hand, in recent years, attempts to simultaneously achieve an improvement in linear recording density and an improvement in surface recording density due to an increase in track density have also continued to be made. In the latest magnetic recording and reproducing apparatus, the track density reaches 400 kTPI.
However, there is a tendency for a problem to occur in that a gradual increase in track density causes pieces of magnetic recording information between adjacent tracks to interfere with each other, and a magnetization transition region in the boundary region thereof acts as a noise source to thereby damage the SNR. This leads directly to a deterioration in the bit error rate (BER), resulting in an obstacle to an improvement in recording density.
In order to increase the surface recording density, it is necessary that the size of each recording bit located on the surface of the magnetic recording medium be made finer, and that saturation magnetization and magnetic film thickness of each recording bit be as large as possible. On the other hand, a problem occurs in that when the recording bit is made finer, the magnetization minimum volume per bit is reduced, and recorded data is lost by magnetization inversion due to thermal fluctuation.
For example, when the recording density is equal to or greater than 2 Tbpsi, an area occupied by 1 bit is reduced to 322 nm2, and when the recording density is attempted to be thermally stabilized, the number of particles for maintaining a signal-to-noise ratio (SNR) required in the magnetic recording and reproducing apparatus cannot be secured. On the other hand, when magnetic particles are made finer in order to maintain the SNR, recorded magnetic data cannot be maintained by thermal instability due to a reduction in volume.
In addition, a gradual increase in track density causes distances between tracks to come closer to each other, and thus an extremely high-accuracy track servo technique is required in the magnetic recording and reproducing apparatus. Generally, in the magnetic recording and reproducing apparatus, a region which a track servo is applied for is wide during recording, and during reproducing a region which a track servo is applied for is narrower than that during recording in order to eliminate an influence from adjacent tracks insofar as possible.
However, when such a method is used, the influence of adjacent tracks can be minimized, but a sufficient reproductive output is not likely to be obtained. As a result, there is a problem in that it is difficult to secure a sufficient SNR.
As one method of solving such a problem of thermal fluctuation, securing a sufficient SNR, and securing a sufficient output, an attempt to increase track density is performed by forming irregularities along tracks on the surface of a magnetic recording medium, and physically separating recording tracks from each other (see, for example, Japanese Unexamined Patent Application, First Publication No. 2004-164692).
Such a technique is generally called a discreet track method. In addition, a magnetic recording medium having track patterns which are magnetically separated from each other in this manner is called a discreet track medium.
Further, for the purpose of an improvement in recording density, a method is proposed in which separation of each magnetic particle is performed even in the longitudinal direction (circumferential direction) of the track, and one magnetic particle is recorded as 1 bit. The magnetic recording medium having patterns in which both the tracks and the bits are magnetically separated from each other is called a bit pattern medium (see, for example, Japanese Unexamined Patent Application, First Publication No. 2005-166240).
In a bit pattern medium, since magnetic interaction between the tracks and between the bits in a longitudinal direction can be suppressed, it is possible to enhance the stability of recorded data. In addition, since 1 bit is constituted by a single magnetic particle, it is possible to suppress transition noise from the disorder of a boundary, and to improve the SNR. As a result, denser magnetic recording can be performed.
In addition, heat-assisted recording in which a magnetic recording medium is irradiated with near-field light or the like to locally heat the surface thereof and in which writing is performed by decreasing the coercive force of the medium is attracting attention as a next-generation recording method (see, for example, Japanese Unexamined Patent Application, First Publication Nos. 2005-166240 and 2006-059474). Meanwhile, Japanese Unexamined Patent Application, First Publication No. 2005-166240 discloses a technique for multi-valuing a cell including both of an upper and lower magnetic layer.